Filling Up the Stetson

Urinating in an open container while riding in a moving car. The phrase originated in 1835, when settlers riding the Oregon Trail were forced to relieve themselves in a large Stetson hat, to avoid stopping their wagons for fear of Indian attacks.

"Geez Jimmy, would you drive a little smoother? I'm filling up the stetson right now, but since you're driving like a maniac, it's splashing around everywhere."